Explosion-proof and firedamp proof headlights generally consist of a battery casing which is carried on a belt, a headpiece worn on the helmet and a cable connecting both. In most cases, the battery casing is designed for receiving two or three nickel-cadmium accumulators or lead accumulators and the headpiece is provided with a 2.4 to 3.75 V incandescent bulb. The accumulators are replaceably inserted into the compartments of the battery casing and serve exclusively for the purpose of lighting.
Headlights of this type are also known to accommodate sometimes other electrically operated equipment, such as two-way radio communicators, gas detectors, pagers, remote warning devices, dosimeters, dustproof helmets etc. by means of a special cable connecting them in an explosion-proof manner with the accumulators housed in the battery casing. This arrangement, however, has a number of disadvantages. First, the electrically operated units cannot be easily interchanged. Second, both lighting and accessories depend on a common source of electric supply as there is only one low-voltage current source available and therefore both circuits may break down in the event of a disturbance in only one of them. Furthermore, reasons of safety determine the need for costly protective devices to interrupt the flow of the current to the accessory in favor of the light should an accident occur and an overload threaten the battery.
The electric power required for the operation of the measuring instruments and communication equipment of the described type was hitherto provided by means of special explosion-proof and firedamp-proof batteries whose casing was equipped with plug connections for said accessories. These batteries, however, were extremely burdensome as they had to be carried in addition to the headlight batteries either hanging from the belt or in some other way. They were wired for automatic safety (had a socalled intrinsically safe output circuit). The prevailing standards of automatic safety for installation of this type (Sch)i require that the short-circuit output of an electric circuit or power supply located in an environment endangered by firedamp be limited to 12 W maximum. This means that in the event of a short-circuit, a current having a nominal potential of 12 V may flow at a maximum rate of 1 A. The battery is provided for this purpose with a protective wiring arrangement which controls the rate of discharge. It is also mandatory that both battery and protective safety wiring be encased. This, therefore, provides the intrinsically safe output circuit.